Optoelectronic devices using organic materials are increasingly desirable in a variety of applications for a number of reasons. Materials used to construct organic optoelectronic devices are relatively inexpensive in comparison to their inorganic counterparts, thereby providing cost advantages over optoelectronic devices produced with inorganic materials. Moreover, organic materials provide desirable physical properties, such as flexibility, permitting their use in applications unsuitable for rigid materials. Examples of organic optoelectronic devices comprise organic photovoltaic cells, organic light emitting devices (OLEDs), and organic photodetectors.
Photovoltaic devices convert electromagnetic radiation into electricity by producing a photo-generated current when connected across a load and exposed to light. When irradiated under an infinite load, a photovoltaic device produces its maximum possible voltage, the open circuit voltage or Voc. When irradiated with its electrical contacts shorted, a photovoltaic device produces its maximum current, I short circuit or Isc. Under operating conditions, a photovoltaic device is connected to a finite load, and the electrical power output is equal to the product of the current and voltage. The maximum power generated by a photovoltaic device cannot exceed the product of Voc and Isc. When the load value is optimized for maximum power generation, the current and voltage have the values Imax and Vmax respectively.
A systemic disadvantage with present organic photovoltaic apparatus is the inability to generate high photovoltages. This inability can limit or preclude use of organic photovoltaic devices in high voltage applications, such as electrochromic or electrochemical applications. To address this deficiency, tandem or multiple-junction organic photovoltaic devices have been developed. While exhibiting enhanced performance, multiple-junction devices are difficult and time consuming to fabricate. Such fabrication inefficiencies can render multiple-junction organic devices cost prohibitive compared to competing inorganic photovoltaic technologies.